1. Field Of The Invention
The present invention relates to a method for evaluating the virus-removing capability of a porous polymeric membrane module for removing viruses by filtration. More particularly, the present invention is concerned with a method for evaluating the virus removing capability of a porous polymeric membrane module for removing viruses by filtration from a virus-containing fluid, which method comprises filling a space on one side of the membrane of the module with a liquid, supplying another space on the other side of the membrane with a gas, terminating the supply of the gas when the transmembrane pressure on the membrane reaches a predetermined level (higher than a level at which non-visible bubbles begin to form but lower than level at which visible bubbles begin to form), allowing the module to stand to thereby cause the transmembrane pressure to be lowered, and measuring the transmembrane pressure lowering a predetermined period of time after the termination of the supply of the gas. By the method of the present invention, the capability of a module to remove viruses can be effectively and efficiently evaluated, thereby enabling the selection of a module which has at least a predetermined level of virus-removing capability and which can be suitably used for removing viruses from a fluid which may contain a virus, for example, protein solutions, such as a plasma, a plasma fractionation product, a culture medium used for cell culturing, and biological pharmaceutical products.
2. Discussion Of Related Art
In recent years, separation techniques using a polymeric membrane have made marked progress and have been used in a wide variety of application fields. The separation techniques using a polymeric membrane are classified into two types according to the principal mechanism of separation, that is, separation by filtration based on the difference between the pore diameter of a porous membrane and the size of a substance to be removed, and separation based on various physical and chemical interactions, such as an adsorption etc., between a membrane and a substance to be removed.
In removing viruses from a virus-containing fluid, however, a porous polymeric membrane module employing the former type of separation technique (filtration) is especially advantageously employed.
With respect to the separation technique for removing viruses from a virus-containing fluid by filtration using a porous polymeric membrane module, various proposals have been made. Examples of such separation techniques are disclosed in Japanese Patent Application Laid-Open Specification Nos. 60-142860, 60-142861 and 61-168367 (in each of which a porous polyolefin membrane is used); U.S. Pat. Nos. 4,808,315 and 4,857,196 and Japanese- Patent Application Laid-Open Specification Nos. 61-254202 and 61-274707 (in each of which a porous regenerated cellulose membrane is used); and Japanese Patent Application Laid-Open Specification No. 62-266072 (in which a porous substance comprising calcium phosphate as a main component, is used).
Meanwhile, since the virus-removing capability of a porous polymeric membrane module is largely influenced by the pore diameter of the membrane, the selection of a porous polymeric membrane module having a predetermined virus-removing capability, has conventionally been conducted mainly by the measurement of the pore diameter of the membrane. Thus, many methods for measuring the pore diameter of a porous membrane have conventionally been known. However, as will be described below in detail, none of the conventional methods are satisfactory as a method for measuring the virus-removing capability of a porous polymeric membrane module for removing viruses.
For example, the mercury intrusion method is known. However, in the mercury intrusion method, the measurement of the pore diameter of a porous polymeric membrane (the diameter is of a submicron order) requires application of an extremely high pressure to a porous polymeric membrane, so that the porous membrane is likely to be damaged. Therefore, this method is unsuitable for the measurement of the pore diameter of a porous polymeric membrane module for removing viruses.
Further, the so-called bubble point test is known in which a first space on one side of a porous polymeric membrane of a module is filled with water and a second space on the other side of the membrane is supplied with a gas and the transmembrane pressure at the time when clear formation of visually observable bubbles occurs, is measured. However, in the case of a porous polymeric membrane having a pore diameter of 100 nm (=0.1 .mu.m) or less, such as membranes for use in removing viruses, the measurement of the pore diameter by the bubble point test requires application of a pressure as high as 30 kg/cm.sup.2 or more, so that the membrane is likely to be damaged. Therefore, this method is unsuitable for the selection of a porous polymeric membrane module for removing viruses.
There is also known a method in which the measurement of the pore diameter is conducted by observing pores through an electron microscope. This method has an advantage in that the pore diameter of individual pores can be directly and accurately measured. However, this method has the following serious disadvantages. That is, the greater the magnification of the electron microscope, the more the area of a portion which can be observed is limited. In general, for obviating this disadvantage, it is necessary that electro photomicrographs of numerous portions of the membrane be taken, thus causing the procedure to be extremely cumbersome. Further, it is impossible to measure the diameters of all of the vast plurality of pores of a membrane, so that measurement of pore diameters cannot be conducted with respect to the entire membrane. Therefore, this method cannot be practically used.
As a relatively practical method, a method is known in which the rate of permeation of water through a porous membrane is measured, to thereby determine the average pore diameter of the membrane. However, with this method, it is impossible to measure a distribution of pore diameters of the membrane, so that the ratio of pores having a pore diameter larger than the average pore diameter and the magnitude of the difference from the average pore diameter (both of which have a great influence on the performance of a membrane) cannot be determined. Therefore, this method cannot be satisfactorily used for the selection of a porous polymeric membrane module for removing viruses.
In the field of filters for removing bacteria, several testing methods called "integrity tests" have been developed and used as methods for examining the capability of a filter to remove bacteria.
Among the integrity tests are a method in which a bubble point is measured as described above, and a diffusion method in which the degree of diffusion of a gas into a liquid through a membrane at a transmembrane pressure at which no bubbles are generated, is measured. The diffusion method can be further classified into a forward flow test in which the flow rate of a gas through a membrane while supplying the gas is measured, and a pressure hold test in which the gas supply is terminated at an appropriate transmembrane pressure and then, a transmembrane pressure lowering is measured a predetermined period after the termination of the supply of the gas (see, for example, "Field experience in testing membrane filter integrity by the forward flow test method", by Wayne Pauli, Ph. D., published by Pall Corporation, Glen Cove, N.Y., U.S.A.). Thus, these integrity tests can be roughly classified into two types of methods, that is, a method (bubble point test) in which a transmembrane pressure at which visually observable bubbles are generated, is measured, and a method (diffusion test) in which the degree of diffusion of a gas into a liquid through a membrane at a transmembrane pressure at which no bubbles are generated, is measured. Practically, the most suitable measuring method is selected, taking into consideration various factors, such as the porous structure, uniformity, strength and production method of the membrane to be examined.
In each of the above-mentioned integrity test methods for examining the capability to remove bacteria, water is mainly used as a liquid. In general, water is suitable as a liquid for use in testing a filter for removing bacteria, which filter has a maximum pore diameter as large as 0.5 .mu.m or more. However, since water has a high surface tension, when water is used as a liquid in the measurement of the virus-removing capability of a module for removing viruses (in such a module the maximum pore diameter of the membrane is as small as 0.25 .mu.m or less), the transmembrane pressure at which the measurement is conducted is inevitably too high, thus damaging the membrane contained in the module. Therefore, the above-mentioned integrity tests for bacteria-removing filters cannot be used for evaluating the virus-removing capability of a porous polymeric membrane module for removing viruses.
Thus, there have been no conventional testing methods which can be effectively and efficiently applied to the evaluation of the virus-removing capability of a porous polymeric membrane module for removing viruses without the danger of damaging the porous polymeric membrane.
In these situations, a novel method for evaluating the virus-removing capability of a porous polymeric membrane module for removing viruses has been earnestly desired.